Sexually transmitted infections (STIs) affect 340 million new people each year worldwide, including an estimated 20 million new infections in the US, adding an estimated $17 billion to US national healthcare costs per year. This application requests renewal of a bioengineering project that began in 1990. Our overall objective is to evaluate the ability of molecules, particles, and cells to move in mucosal tissues and to use that understanding to design methods for the controlled delivery of agents to prevent STIs. During the period since our last competing application, we completed a wide-ranging series of studies designed to show the potential for controlled drug delivery systems in this setting. We showed that: 1) antibodies, DNA, RNA, and peptide nucleic acids can be released slowly from biocompatible polymers, including nanoparticles (NPs), and these agents are active at mucosal surfaces; 2) surface-modified NPs penetrate readily through unstirred human cervical mucus; 3) vaginal rings releasing protein antigens can stimulate long-lasting, mucosal immunity; 4) polymer NPs loaded with siRNA delivered to the vaginal mucosal surface are capable of knocking down genes that interfere with HSV transmission in mice; and 5) NPs loaded with siRNA directed against SIV genes reduce viral loads in chronically infected macaques. In this renewal period, we exploit the experience we have gained in past studies, by presenting a focused plan to develop a safe and effective topically-administered NP delivery system for prevention and treatment of HSV-2 infections. The proposed work builds on what we have learned over past grant periods and provides a clear route for translation of one of our most significant innovations to clinical practice. We will accomplish our goal in three specific aims: First, we will optimize multifunctional NPs for topical siRNA delivery in the female reproductive tract. The multifunctional NPs will be based on our biocompatible PLGA delivery systems with additions to improve effectiveness and safety: cationic polymers will enhance siRNA loading and endosomal escape; PEG will facilitate mucus penetration; and shedding of PEG will facilitate vaginal retention and cellular uptake. We will test efficiency, duration, and toxicity o these multifunctional NPs after vaginal administration in mice. We will also examine the toxicity of our best preparations in non-human primates. Second, we will determine the effectiveness of these multifunctional NPs for prevention of HSV-2 infections. Third, we will measure the effectiveness of these NPs for treatment of recurrent HSV-2 infections. These studies will use state-of-the-art animal models to test a new method for prevention of HSV-2 transmission and treatment of recurrence.